Given the clinical relevance of the changes in gene expression induced by dysregulation of RRP1B, our current work focuses upon defining the mechanism by which Rrp1b regulates metastasis-associated transcription. This has primarily been achieved by defining the RRP1B transcriptome using a combination of ChIP-seq and RNA-seq. ChIP-seq performed within the last year in a variety of cell lines has revealed that RRP1B interacts with multiple genomic loci to regulate gene expression. Our analyses reveal that RRP1B is regulating gene expression through a variety of mechanisms. One particularly relevant mechanism of regulation is suppression of gene expression by RRP1B-mediated binding of the TRIM28/HP1-alpha heterochromatin complex. This is associated with an increase in histone H3 tri-methyl lysine 9 levels at RRP1B bound genomic loci, implying that RRP1B binding induces changes in epigenetic markers. Work in the following year will focus upon defining further mechanisms by which RRP1B alters gene expression and the implications of this upon metastasis in breast cancer. Additionally, RNA-seq has revealed that RRP1B regulates alternative mRNA splicing, a process which is ubiquitously dysregulated in advanced tumorigenesis. Specifically, we have demonstrated that RRP1B knockdown inducing differential isoform expression in over 600 genes. This appears to be mediated through a transcriptionally-dependent interaction with the splicing regulator SRSF1. Given that transcription and splicing occur concurrently, and RRP1B interacts with both transcription and splicing-associated factors, it appears that RRP1B regulates gene expression at multiple levels to modify metastasis. We believe that studies such as this will give us greater insight into origins of metastasis-related gene expression programs and increase our overall understanding of this most deadly consequence of breast cancer. Work in the coming year will utilize these findings to determine whether RRP1B is a potential therapeutic target in advanced breast cancer. Our work with NDN is at a less advanced stage. We have identified a non-synonymous coding variant of mouse NDN that is present in a number of laboratory mouse strains. While ectopic expression of wildtype NDN suppresses metastasis, the converse is true of variant NDN, with a profound up-regulation of metastasis observed in metastatic mammary tumor cell lines ectopically expressing this variant. This particularly interesting observation will be one of the central focuses of our analysis of the NDN transcriptome, again using ChIP-seq and RNA-seq. These studies will allow us to extend our observations to focus upon the function of human NDN and its putative role in human breast cancer progression and metastasis.